The detection of the position of a movable object using electromagnetic radiation is known in the prior art. These systems usually contain both a source and a detector and the position of the movable object is determined by the absence of a normally detected signal. Such systems typically used light radiation and the source emits a beam of light which is detected by the detector. The position of the movable object is known when the light beam is broken by the object moving between the source and the detector. An example of a photoelectric means for detecting the presence of a vehicle in a parking garage is disclosed in the U.S. Pat. to Inuzuka et al, No. 3,323,617, incorporated herein by reference.
It is also known to use non-visible light electromagnetic radiation, ultrasonic radiation, and higher frequency electromagnetic radiation such as used in radar and microwave transmission systems. For example, the U.S. Pat. to Cheal et al, No. 3,877,002, incorporated herein by reference, discloses an intrusion detection system using microwaves.
There are a number of disadvantages to each of the prior art systems when they are used to locate when a movable object is at a predetermined position. Firstly, the use of laser beams or other light frequency electromagnetic radiation systems require very accurate alignment between the source and the detector and, even when aligned, do not provide the necessary degree of accuracy unless the movable object is specially configured. Systems using light radiation also are prone to indicate improper detection or are otherwise unreliable when the environment between the source and the detector becomes polluted. Such pollution can include fog, smog, smoke, and other light reflecting substances. Therefore, in certain environments such as found outside an enclosed structure, light radiation systems are unreliable.
The conventional systems which use ultrasonic radiation and radar and microwave radiation suffer from other disadvantages. For example, such systems can suffer from the problem of stray radiation reaching the detector, which problem becomes more pronounced as the distance between source and detector increases. In addition, it has been found by the present inventors that ultrasonic systems used outside an enclosed structure are subject to drift, environmental noise pollution, and other degradation from climatic conditions. It was even found that the sound beam was broken by a strong wind blowing the transmitted ultrasonic sound beams away from the detector.
When it is necessary to determine when each one of a plurality of movable objects is in a predetermined position, such as when each tray of an endless conveyor is at a loading position, the detecting system can become prohibitly expensive if that system requires duplication of all of the components on each movable object. On the other hand, it is sometimes desirable to know not only when the movable object is in a predetermined position, but also when a further object, carried by the movable object, is properly positioned on the movable object. More specifically, in the field of automobile parking systems (such as those disclosed in U.S. Pat. to Coursey, No. 2,874,822; Cooper et al, No. 2,578,046; Holappa No. 2,773,609; and Lichti, No. 3,656,608; all of which are incorporated herein by reference) it is necessary to know when the automobile is properly positioned on the supporting carriages, cradles, or conveyor pans. Both the Coursey patent and the Cooper et al patent disclose the use of conventional mechanical switches which are depressed by the tire of the automobile when the automobile is properly positioned. The compounding of costs is clearly disclosed by the duplication of components on each carriage 36 in the Coursey patent. The system of this patent also requires a complex series of contacts at each rest location in order to sense the signal generated on the movable carriage platform. In addition, conventional electromechanical switch systems are obviously prone to failure and breakage and other malfunctioning as a result of the accumulation of debris and other material inside the switch. Such a system with its complexity and large number of items can produce a maintenance nightmare.
Therefore, it is believed that none of the known conventional systems provide a reliable and simple position detector. Furthermore, none of the conventional systems provide a detector which is usable outside of enclosed areas or in areas where there is a large distance between the source and the detector.